Sounds like a cartridge filter. It should be second piece of equipment after the pump. There's usually a pressure gauge on the top. If he removes the lid of this equipment, it will either be a cartridge (a round pleated element), DE (white powder on a cloth type grid), or sand (such as yours). Every equipment set has to have a pump and filter. The rest are optional.

Don't get into the Nature 2 thing. It "may" help, but is not necessary with the salt sytem. Yes, the salt and zeobrite is a good combination.